1. Field of the Invention
The invention relates to a high pressure mercury lamp, and especially to a high pressure mercury lamp with high radiance which is used as a light source for back lighting of a liquid crystal projector and for fiber illumination.
2. Description of Related Art
In a liquid crystal display device of the projection type there is a need for illumination of images on a rectangular screen in a uniform manner and with adequate color reproduction. Therefore, as the light source, a metal halide lamp is used which is filled with mercury and metal halides. These metal halide lamps have recently been made even smaller so that more and more they represent point light sources. Metal halide lamps with an extremely small distance between the electrodes are used in practice.
Proceeding from this background, instead of metal halide lamps, recently, lamps have been suggested with a mercury vapor pressure which is higher than ever before, for example, greater than or equal to 200 bar (roughly 197 atm). Here, by increasing the mercury vapor pressure, spreading of the arc is suppressed (concentrated), and furthermore, there is an effort to increase light intensity even more. These lamps are disclosed, for example, in Japanese patent disclosure document HEI 2-148561 and Japanese patent disclosure document HEI 6-52830.
In Japanese patent disclosure document HEI 2-148561 (U.S. Pat. No. 5,109,181) a high pressure mercury lamp is disclosed in which a discharge vessel provided with a pair of tungsten electrodes is filled with a rare gas, greater than or equal to 0.2 mg/mm.sup.3 mercury, and a halogen in the range from 1.times.10.sup.-6 to 1.times.10.sup.-4 .mu.mole/mm.sup.3, and which is operated with a wall load of at least 1 W/mm.sup.2. The reason for adding an amount of mercury of at least 0.2 mg/mm.sup.3 is to improve color reproduction by increasing the mercury pressure and the continuous spectrum in the area of visible radiation, especially in the red range. The reason for a wall load of at least 1 W/mm.sup.2 is the need for a temperature increase in the coolest portion in order to increase the mercury pressure. The reason for adding the halogen is to prevent blackening of the envelope; this can be taken from the publication. The reason for fixing the halogen in the range from 1.times.10.sup.-6 to 1-10.sup.-4 .mu.mole/mm.sup.3 is, however, not described. Furthermore, it is also described that the halogen cannot be added in the form of a metal compound because this would etch the electrodes.
On the other hand, in Japanese patent disclosure document HEI 6-52830 (U.S. Pat. No. 5,497,049), it is described that, in addition to the above described amount of mercury, values of wall load and amount of halogen, the shape of the discharge vessel and the distance between the electrodes are fixed, and furthermore, the type of halogen is limited to bromine. The reason for adding bromine is to prevent blackening of the envelope. When at least 10.sup.-6 .mu.mole/mm.sup.3 bromine is added, a sufficient effect is obtained. It is also shown that when more than 10.sup..sup.-4 .mu.mole/mm.sup.3 bromine is added, the electrodes are etched. Furthermore, it is described in this publication that this lamp is suitable for a projector light source and that the degree to which illuminance of the image surface of a liquid crystal projection television is maintained is better at 4000 hours than in a conventional lamp.
However, in the above described conventional lamps, it was considered disadvantageous that the arc fluctuates during lamp operation. The reason for this is not entirely clear, but the following is assumed.
Since the amount of mercury added is high, the mercury vapor pressure is extremely high. Consequently, the arc contracts and becomes very narrow. Since a large amount of power is being supplied to this narrow arc, so that a large tube wall load results, the power density in the arc is therefore extremely high and the arc temperature rises. Due to the extremely high mercury vapor pressure, the narrowness of the arc, and the extremely high temperature, the speed of convection in the arc vicinity is greatly increased. The temperature on the boundary between the arc and the peripheral area is steeply changed and as a result arc fluctuations presumably occur.
In the above described publications of the prior art, it is furthermore described that the emission of the portion of red is increased and that emission with sufficiently good color reproduction can be effected. In the case of use as the background light of a liquid crystal projector, however, it cannot be stated that the need for high color reproduction has been adequately satisfied recently. This means that there is a need for emission in which the portion of red is increased even more.